The field-effect-transistor (FET, or transistor) uses either electrons (in N-channel FET) or holes (in P-channel FET) for conduction. The four terminals of a transistor are source, gate, drain, and body (substrate). In transistors, the drain-to-source current flows via a conducting channel that connects the source region to the drain region. The conductivity is controlled by the electric field that is produced when a voltage is applied between the gate and source terminals, denoted by Vgs. Usually, the body terminal is connected to the highest or lowest voltage within the circuit. The body terminal and the source terminal are sometimes connected together since the source is also sometimes connected to the highest or lowest voltage within the circuit. Normally, an input signal is applied to the gate terminal of a transistor, and an output signal is connected to the source or the drain terminal of a transistor. A first terminal of a transistor can be either its source terminal or its drain terminal, and a second terminal of a transistor is the drain or source terminal of the transistor.
A digital circuit accepts input signals and produces output signals, both could be represented by certain allowed voltages. A flip-flop (a latch) is a circuit that has two stable states and can be used to store state information. The latch circuit can be made to change state by signals applied to one or more control inputs and will have one or two outputs.
In digital circuits, a logic level is one of a finite number of states that a signal can have. Logic levels are usually represented by the voltage difference between the signal and ground (or some other common reference point), although other standards exist. The range of voltage levels that represents each state depends on the logic family being used. An active-high signal represents a binary digit of 1, or asserted state of a logical condition, by the higher of two voltages. An active-low signal represents a binary digit of 0, or asserted state of a logical condition, by the lower of two voltages. In three-state logic, an output device can also be high impedance. This is not a logic level, but means that the output is not controlling the state of the connected circuit. A level shifter connects one digital circuit that uses one logic level to another digital circuit that uses another logic level.
Manufacturers have developed different processes to produce Integrated Circuits (IC) that operate at different voltage levels. Some common IC operating voltage levels include 5V+/−10%, 3.3V+/−10%, and 2.5V+/−10%. In using decreased voltage levels, manufacturers limit the adverse effects of power dissipation (e.g., heat), while continuing to allow for ever increasing IC densities.
Nevertheless, when a new, low-voltage IC process technology emerges, it is often desirable for the new technology to be able to operate with existing high-voltage levels. IC process technologies, and their respective operating voltages, are often defined by the gate-oxide breakdown voltage between the terminals of a device (e.g., a transistor) implemented using the particular process technology. Consequently, a potential problem with interfacing circuitry implemented in a low-voltage process technology with a voltage that exceeds device limits is that, one or more devices implemented in the low-voltage process may experience damage, either temporary or permanent, that can hinder the circuit's ability to perform its desired function.
A voltage level shifter can function as a high-voltage tolerant output driver providing the ability to regulate an input voltage VIN that may exceed the maximum operating voltage of the process technology. Without high-voltage tolerant output driver, exceeding the device voltage limits dictated by the process technology may result in damage of devices.
The drawings, schematics and diagrams are illustrative and not intended to be limiting, but are examples of embodiments of the invention, are simplified for explanatory purposes, and are not drawn to scale.